In search of a cure for a sickness that's so far claimed six billion lives, scientist Nyssa arrives at an abandoned gene-tech facility on the toxic planet Helheim. 'Hellhole', more like.

Nyssa's not alone. The TARDIS has also been drawn to the Helheim base — and in its cobweb-coated corridors, she soon runs into the Doctor, Tegan and Turlough, her travelling companions of half a century past. But who, or what, has engineered this strange reunion? The Black Guardian, perhaps?